This invention relates to plastic injection molding machines and more particularly to a plasticator/injector unit for injection molding machines having novel valving and material feed arrangements.
In injection molding raw stock usually in the form of beads or granules of plastic or polymeric material is fed into a plasticator that may take the form of a screw-like element rotating in a long heated cylindrical barrel. As the screw element is rotated the feed stock material is moved through the barrel under the pressure of the screw element while being heated to its molten state. As the screw element continues its rotation the molten plastic is then pumped under relatively moderate pressure into the chamber of an injection cylinder. When the chamber is fully charged, the molten plastic therein is injected into a mold from the chamber under extremely high pressure.
In some types of plasticator injector machines, an accumulator chamber is used to hold the molten plastic extruded from the screw plasticator until enough has been accumulated to fill the chamber of the high pressure injection cylinder. The plastic must be injected into the mold at very high pressure.
The injection molding process is cyclical in that after the high pressure injection of plastic into the mold a finite amount of time it is required to remove the product from the mold or otherwise provide an empty mold to receive the next injection of plastic. However, the plasticating process feeding the cyclical mold process is most efficient as a continuous process that does not require the stopping and starting of the plasticator unit. Such stopping and starting results in uneven heating and treatment of the plastic. Thus, it is a usual practice in plasticator/injector machines to provide a means to accumulate the molten plastic emerging from the plasticator in an amount sufficient to charge the high pressure injection cylinder with enough plastic to fill the mold when injected under high pressure.
Injection of the plastic into the mold must be carried out under extremely high pressure and thus it has been found advantageous to isolate the plasticator and accumulator units from the injection chamber during the injection process so that only the injector part of the machine must be made to withstand the very high injection pressures. Since only the process of injecting the plastic into the mold requires very high pressure, the plastic can be transferred from the accumulator into the injection cylinder under a relatively moderate pressure, for example, a pressure only about equal to the pressure produced by the plasticator screw in extruding the molten plastic.
Some machines now in use provide a reciprocating inline screw element in the plasticator inline with the injection chamber so that plastic accumulates in a chamber created as the screw element moves back through the cylinder. Then as the screw is moved forward the accumulation of plastic is transferred through one way valving into an injection chamber. Extremely high pressure is then used to inject the plastic of the chamber into the mold with the one way valve blocking the high pressure from the plasticator and accumulator areas.
In another type of injection molding machine, a reciprocating screw-type plasticator is arranged inline with the injection channel to the mold and a side chamber is used as the injection chamber which is filled by plastic accumulated in front of the plasticator screw as it is moved back into the plasticator. A rotary shutoff valve is used to isolate the plasticator from the extremely high pressure injection of plastic from the side chamber. After injection, the rotary valve is opened and the screw is moved forward to pump additional plastic into the injection cylinder.
In still another form of injection molding machine, a non-reciprocating screw type plasticator is arranged off axis from the injector cylinder and extrudes plasticized material into a side chamber buffer reservoir. When sufficient plastic has accumulated in the reservoir chamber it is forced out of the reservoir by either piston or gas pressure through a valved channel or conduit into and an injection cylinder chamber. The conduit valve when closed isolates the buffer chamber and plasticator from the high pressure of injection.
Examples of the above described injection molding machines are shown in U.S. Pat. No. 2,581,477 to G. Triulzi, U.S. Pat. No. 3,395,424 to R. Nouel, U.S. Pat. No. 4,557,683 to G. Meeker et al, U.S. Pat. No. 4,695,238 to Y. Taniguchi and U.S. Pat. No. 4,784,819 to R. Spurr.
Machines of the type above described have been found to have certain disadvantages. For example, the mechanisms required in the reciprocating screw machines have added to the complexity and consequently the cost of those machines. The cut-off valves used to isolate the accumulator sections from the high pressure injection sections in some machines tend to leak around their control shafts dripping plastic on and in the machines. In addition, in many machines the plastic material may not be uniformly heated or otherwise treated either because the plasticator screw must be stopped periodically while material is transferred or ejected from the accumulator or injection chambers. Non-uniform treatment of the mateiral may be because of material transfer systems wherein the first portion of the material entering a chamber is the last portion to leave during transfers resulting in unequal heating and treating times.